Bear in mind though that if you have two vouchers, you can't use one for two people outbound and the other for two people for the return, as they are, according to the T&Cs, only valid from the UK.

Mind you, last week I received an email from Barclays saying the following:

"A complimentary British Airways cabin upgrade voucher1
Make the most of your overseas adventure with a cabin upgrade voucher. With your voucher, you’ll be able to upgrade by one cabin class on an Avios Reward flight booking – either on an individual roundtrip journey (outward and return), or for two people, one way (either outward or return)" (emphasis added).

I wonder if I could require them to let me book a one way return for two (i.e. from outside the UK) on the basis of that?